FE-1230-3 FE-1230-3

Dis. Category UC-90-g

DEVELOPMENT, TESTING AND EVALUATION OF

MHD-MATERIALS

Quarterly Report

for the period Jan. - March, 1976

H. P. R. Frederikse, T. Negas and S. J. Schneider

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Date Published: March 31, 1976

PREPARED FOR THE UNITED STATES

ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

Under Contract No. E (49-18) -1230

IMPORTANT NOTICE

NATIONAL BUREAU OF STANDARDS REPORTS are usually prelimmarv or progress accounting documents intended

lor use within the Government. Before materul in the reports is formally published it is subjected to add.lionji evaluation

• nd review. f :i this reason, the publication, reprinting. reorDduclion. or open iteralure listing of this Reoort, either in

whole or ir> part, is not authpfi:;d unless permission is obtained in wntin? trom the Oldce of the Director. National

Bureau of Sta-- Jards. Washmg'cn. D C. Z0234. Such permission is not ricedcd. however, by the Government agency loi which

the Report has ocen spccmcally prepared il tnat agency wishes to leproducc additional copies (or its own use.

ABSTRACT

During this quarter activities were focused on the characterization of

the electrode materials tested in the U-02 MHD facility in September 1975

(Phase I). A Final Repc rt about all aspects of this test, including

contributions from Westinghouse , Battelle Northwest, and from the Soviet

team, is in an advanced stage of preparation.

Conclusions derived from the post-test evaluation are being used to

plan Phase II of the U-02 Materials Testing Program. During a meeting of

the US-USSR Working group - in which several NBS staff members participated

it was decided that mid-August will be the target date for the next U-02

te.*5t.

In this quarter, collection of data on struct\xral, chemical and

electrical properties of spinels has been emphasized, because these

substances show great promise as electrode materials; one or two of

them probably will be tested in the U-02 facility this summer.

In February ERDA requested the Bureau to assist in the evaluation of

channel wall materials after a 95 hour test of the AVCO MHD generator.

A number of anode and cathode elements have been received and prepared

for optical. X-ray and SEM experiments.

Other activities listed under Tasks I, J and L are being continued.

The \7ork on viscosity of coal slags is concerned with the effect of various

redox conditions. Vapor pressures of over slags of different compositions have been measured and analyzed. SEM studies of K-diffusion into

alumina are now beyond the "try-out" stage. The first phase of the

assessment of downstreaun components, defining performance requirements,

has been completed; efforts are now being concentrated on the problem

of metal alloy corrosion by potassium vapor.

1

Objectives and Scope of Work

The overall objective of this program is to obtain chemical and physical

definition of high temperature materials which have shown promise for use

in coal-fired open-cycle MHD power systems. Major problem areas in which

investigations will be concentrated are:

1. Characteristics of coal slag and its effects on system components

and performance at prototype temperatures.

2. Development of electrode materials which provide adequate performance

over extended periods of time.

3. Insulating materials which limit theTonal losses and are resistant

to prolonged thermal and erosion effects.

4. Preheater materials which can withstand the operating modes of

separately and directly fired operation.

5. Seed recovery methods from slag which are technically and economically

feasible.

6. Phase equilibria and diffusion rates of seed in slag and the corrosive

action of combination on system components and materials.

7. Lfurability of prototype MHD sub-systems.

The program is designed to contribute to the solution of these problems

by providing much needed data on candidate materials and by evaluating test

samples and structures that have been subjected to real or simulated MHD

conditions. The activities are grouped under six tasks:

G. Program Management Coordination (Assisting ERDA in coordination,

planning and review of the various MHD-Materials Development Programs)

H. U-02 Materials Testing and Characterization (Coordination of U-02

Test Activities, Phase I)

I. Operational Design Properties (viscosity, electrical conductivity,

vaporization)

J. Corrosion by Seed and Slag (phase equilibria, diffusion)

K. Materials Testing and Characterization !test coordination, pre- and

post-test analysis)

L. Assessment of Steam Plant Components (corrosion resistance of metals

and alloys)

2

Summary

Ta

k G. Program Management Coordination

- Conducted a meeting of the U.S. members of the Working Group for MHD

Materials in January 1975. Participated and chaired sessions of the

US-USSR MHD-Materials Working Group in February 1976. At both meetings

Phases I, II and III of the U-02 Materials Testing Program and the testing

of Soviet MHD-materials in the U.S. were discussed.

Task H. U-02 Materials Testing and Charactei'ization

- Finalized the post- test experiments conducted at NBS (Phase composition,

electrical conductivity, radiography)

- Completed the draft of the U.S. parts of the Final Report describing

the U-02 materials test (Phase I)

Task I. Operational Design Properties

a. Viscosity of Coal Slag

- Iridium parts were received for adaptation of the viscometer to extend

measurement to 1800 °C.

- Preliminary measurements revealed that a reducing atmosphere (by bubbling

"forming gas" through the melt) had no visible effect on the viscosity of

slag. Bubbling of oxygen, however, increased the viscosity at 1370 °C

by about 7%.

b. Electrical Conductivity

- Conductivity measurements on more than a dozen Fe-doped spinels indicated

that some of these substances may be good MHD electrode materials.

c. Vaporization

- Potassium pressures measured over two synthetic six-component slags were

found to be significantly higher than the K-pressures over K^O-SiO^ solutions

(with the same K^O concentration at the same tempei^ature) .

Task J. Corrosion Processes, Seed-Slag Interaction, Diffusion

- Exposure of Mg-Al and Mg-Cr spinels to seed (K^CO^) indicated that the

former shows good corrosion resistance, while the latter reacts readily

with potassium forming K^CrO^ . However, the chromite spinel appears to

withstand attack by slag rather well; in this situation (when the spinel is

covered with slag) the K corrosion will be diminished considerably.

- A refractory ceramic (spinel + MgO) tested by FluiDyne Engineering Corp.

for slag corrosion is being characterized using SEM and EDX techniques.

- Studies of potassium diffusion from (K) 3-alumina into pure ^12^3

being continued.

3

Task K. Materials Testing and Coordination

- Electrical measurements and X-ray diffraction studies are being conducted

on samples of materials that will be tested in the Phase II-U-02 experiment

planned for August 1976. These samples include specimens fabricated at

Westinghouse , Battelle and NBS.

- Electrode sections from the AVCO MHD channel wall that has been tested

in a 95 hour run made in February at the AVCO Everett Laboratories, are

being analyzed and characterized at NBS (X-ray diffraction, SEM, etc)

'Cask L. Assessment of Steam Plant Components .

- Environmental conditions (temperatures, pressures, etc.) are being collected

from experimental data and model calculations performed by other organizations.

These conditions (and hence performance requirements) for components "below"

the preheater (or seed condenser) are constantly updated and modified.

- Reports are being analyzed dealing with the potassium corrosion resistance

of various alloys (stainless steel, Inconel, Hastelloys, etc.) in the range

870 - 1000 °C.

4

ACCOMPLISHMENTS AND MILESTONES

1. Accompli shine nts

Most of the NBS work on MHD-Materials performed under this contract is

closely interwoven wit a the overall ERDA Program plans for MHD-development

and hence, has to be aligned, nearly daily, with the direction in which

these plans proceed. For that reason NBS keeps close contact with the

ERDA-MHD Division and with the various contractors involved in generator

testing, preheater evaluation, MHD system modeling, etc. Often NBS has

to change its plans, put more emphasis or a particular aspect of the program,

shift personnel for a certain period to the particular task that has

highest priority, and consequently decrease the effort temporarily in

other areas. Hence, the design of program plans has to be flexible, projected

goals reconsidered and targets or target dates readjusted.

During the past 9 months, Task K, Materials Testing and Coordination,

has received by far the most attention. Most of the work has been connected,

in one way or another, with the U-02 (Phase I) test conducted during last

September in Moscow. Activities during July, August and September included

the drafting and finalizing of detailed work plans for nhe Battelle Proof

Test and the U-02 test, determination of physical or chemical properties

of selected test electrodes and insulators, preparation and formulation of

Phase I post- test activities as well as Phase II test plans, and finally

participation in the U-02 test. Immediately after the experiment, efforts

have been concentrated on the post-test characterization of materials. This

included disassembly of the modules, recording and photographing of the

various components, slicing of the cathode and anode sub-modules, preparation

of these slices for X-ray diffraction, SEM, electroprobe , radiographic,

theirmophysical , etc. measurements. Around February 1, 1976 a first draft

of the results of these measurements, including conclusions, was compiled.

Slamming up the achievements of the NBS effort during the first ^ months

of the contract in terms of the projected targets (milestones) as formulated

in the original work statement the following completed goals can be listed:

(4a) . Completed the first phase of the study concerned with the effect of

alkali additions on the viscosity of coal slags.

(7). Determined the vapor pressure of K^O in the system K^O-K^SO^-SiO^

(16a) . Determined the 2lectrical conductivity of various coal slags as a function cf temperature and oxygen partial pressure. (The investigation

of the dependence on electric field strength is continuing)

(19) . Measured the ele:;trical conductivity of promising electrode materials

(in particular: 7,rO^:Y ^O^, ZrO^rCeO^, Mg-Cr-Fe spinels and Mg-Al-Fe-spinels) .

(20) . Designed, fabricated and installed equiprient for measuring electrical,

mass-, etc. transport through a composite test electrode in a large

temperature gradient.

(24) . Determined the phase diagram for the systems K 0-ZrO

(27) . Studied the phases and eutectics in the system (K2CO3) -Ce02-Zr02

(28) . Assessed the resistance of CeO^-based materials to seed corrosion.

i (30a) . Evaluated, selected and procured equipment (an AMR Scanning

Electron Microscope and Energy Dispersive X-ray analyzer) for the measurement

of diffusion of seed (and other elements and compounds) into MHD materials.

(31a) . Completed preliminary test experiments aimed at the determination

of the rate of diffusion of certain species (potassium) into alumina,

magnesia, etc. ceramic bodies.

(39) . Coordinated testing and pre- and post-test analysis of specimens

exposed to (simulated or) real MHD conditions in (a variety of test rigs,

arc heaters, small- and) large-scale MHD generators. [U-02 (Phase II)

AVCO, Westinghouse, etc.].

(40) . Used a variety of chemical and physical techniques to assess the

degradation of certain chromites and aluminates tested for specific time

periods in a stimulated MHD preheater test facility (at Fluidyne Engineering

Corp)

(41a) . Analyzed and compiled (from data and experiences of MHD facility

operators) the range of environmental conditions, under which steam plant

components must function.

6

Task G. Program Management Coordination

U.S.-U.S.S.R. Materials Program Activities

During this reporting period two meetings of the Materials Working

Group took place. Four NBS staff members participated: F. J. Schneider

(chairman), H. P. R. Frederikse, VJ. R. Hosier and T. Negas. On January 15

(at NBS) and on January 16 (at ERDA Headquarters) the U.S. Group met for

the purpose of briefing ERDA staff on the results and conclusions of the

U-02 test-Phase I, and furthermore to plan and establish strategy for the

forthcoming meetings (February 1976) with the USSR Materials Group.

The status of the Phase I results and conclusions is described in

another section (Task H) of this report. It was the consensus that plans for

the Phase II, U-02 test (scheduled for June 76) should proceed as originally

communicated to the Soviets during the October 1975 meeting (100-200 hour

test, relevant to U-25 channel development, K2S0^ seed if agreeable to

Soviets) . The main inhibiting factor in meeting the June test was the

lack of time necessary to test, select and prepare candidate test materials.

The primary materials however would be improved (microstructure) Phase I

electrodes. Plans for Phase III were also formulated. This test would be

based on replenishment (Fe-Al-Silicate) and would be condacted through a

joint effort with each side having responsibility for design, construction

and testing.

During the February 1976 meeting with the Soviets the above positions

were negotiated. The U.S.S.R. side agreed to proceed with Phase II plans

as presented by the U.S. side; however, the target date for the test was

shifted to mid-August 1976. Phase III plans were not agreed to entot.o but

major points were acceptable. The Steering Committee at its June meeting

will hear detailed plans from both sides. The Soviets did not transmit all

the documents they originally agreed to (Chapters for the Phase I report and for the Joint Status Report). These will be furnished by May 1976.

The testing of U.S.S.R. materials at Westinghouse and UTSI will begin in the

Fall of 1976, as scheduled. A. J. Armstrong of NBS attended meetings with

the Soviet Materials Group at Westinghouse (Feb. 23) and at UTSI (Feb. 24)

at which the work plans for these tests were discussed.

MHD Workshop on Replenishment

As indicated above plans for Phase III of the US-USSR Materials

Testing Program include the use of a replenishment material. To provide

background and documented information about this aspect of MHD generators

an ERDA sponsored workshop was held at NBS on December 17, 197 5. Thirty-five

persons representing the major institutions and companies involved in MHD

attended this one-day meeting. Ten papers were presented discussing various

replenishment (maintenance) materials and procedures, including zirconia and

fly ash injection. A report covering these presentations and the subsequent

discussions will be published shortly.

7

Task H. U-02 Materials Testing and Characterization (Phase I)

a. Post Test Characterization (W. R. Hosier)

The U.S. portions of the "Final Report on the U-02 Materials Test

(Phase I)" has been completed. A version of Chapter I written by the

Soviet participants, was presented in February to the Editor of the

Final Report (W. R. Hosier) and has been incorporated into the draft

report. Soviet contributions to Chapters IID, III and IV have not yet

been received. The Final Report is divided into four Chapters:

Chap. I. Description of the Test Facility and Operating Conditions

During the Test (USSR)

Chap. II. Module Description, Analytical Procedures and Results of

Analysis (US and USSR)

A. Module Design and Materials Preparation (Westinghouse)

B. Pre-test Characterization

a. Thermal diffusivity, thermal expansion, chemical

composition, microstructure (Battelle)

b. Electrical conductivity, phase composition (NBS)

c. Porosity and pore size distribution (Westinghouse)

C. Post-Test Characterization

a. Module disassembly, phase composition, electrical

conductivity, radiography (NBS)

b. Metallography and electron microprobe analysis (Battelle)

D. Soviet Results on Pre- and Post-test Characterization (USSR)

Chap. III. Comparative Analysis (with previous Soviet tests and test

results) (USSR)

Chap. IV. Conclusions (US and USSR)

The Final Report will contain an extensive description of all stages

of the test; the preparation and checks made before the test, the procedures

followed and measurements performed during the test, and the analysis of

thei materials after the test. For detailed information concerning any

aspect of the test we refer the reader to the Final Report, which will be

available later this year. A paper giving a condensed account of

the entire U-02 test (Phase I) and materials characterization will

be presented by S. J. Schneider at the 15th Symposium on the Engineering

Aspects of MHD (May 24-26, 1976, Philadelphia, PA).

For purposes of this Quarterly Report we will present a few details

involving the items mentioned in Section C.a. of Chapter II.

1. Module Disassembly (W. R. Hosier, A. J. Armstrong)

As discussed in the previous Quarterly Report (December 31, 1975) several

slices of each of the 5 sub-modules were obtained by sawing lengthwise

through the electrode assembly. Figiires la and lb show a photograph of cathode

and anode cross sections of each of the 5 sub-modules. The numbers correspond

to the last or last two digits of the 4-digit identification used for

labeling the electrodes (See Fig. 2).

n



I I

CATHODE

1102

1103

110^

1205

1205

1207

1208

1309

1310

1311

1312

1^13

ms

I^Ig:^;

1517

1518

1519

1520

88 ZrO

2

I2-Y2O3

82 ZrO;

18 CeO

2

50 ZrO^

50 CEO2

25 ZrO;

75 CeO'

r^<0> ZRO9

^1,^ CEO2

2 Ta.^O;

A[^ODE

2101

2102

2103

2104

2205

•

2206

CD

>

<:<<2208;:<:^^ c

2309

2310 m

0

2311

-H0

2312

2413

2414 1

2416

2518

2519

2520

FIG. 2. " SCHEMATIC OF CATHODE AND ANODE SHOWING MATERIALS^

IDENTIFICATION DIGITS AND ELECTRODE LEAD-OUT FAILURE.

SHADED AREAS INDICATE WHICH LEAD FAILED (aNODE OR CATHODE).

11

2. Phase Analysis of Materials (T. Negas, C. McDaniel, A. Perloff)

The results of x-ray phase analysis of electrodes utilized in the U-02,

Phase I test, are summarized in Tables I and II. Several conclusions are

reached based on these data together with a working knowledge of how ZXO2/

cerium oxide materials behave as a function of temperature and Pq^ (see

for example, Quarterly Report, March 31, 1975). Cathode materials containing

Ce02 were subjected to reduction caused by low (partial) oxygen pressure or by

electric field controlled mass transport. It is difficult to separate

these two factors based on this test. Additionally, potassium seed penetrated

the cathode ceramics, probably via cracks, pores and through the surrounding

porous cement. Identification of , pyrochlore (actually reoxidized

during cool-down) plus monoclinic ZrO^ in the materials is a strong indicator

of reducing conditxons , while the compound having the approximate composition

•KGeZi^g (a perovskite) obviously is a "marker" for reaction with K2O.

Reduction processes which may compromise the mechanical properties of pre-test,

oxidized, Zr02/Ce02 electrodes cannot be avoided at the cathode, unless

electrodes are fashioned from pre-reduced materials. However, during the

start-up and cool-down intervals of an MHD system, pre-reduced materials

would be subjected to oxidation unless rapid start and stop procedures are

utilized. Reactions with K2O may be minimized by reducing the porosity,

controlling the cracking (if possible) and, perhaps, by increasing the

backface-leadout temperature. What this optimum temperature should be is

ill-defined although the condensation temperature range (<1400 °C) of seed

provides a rough guide.

Supporting laboratory work together with a more detailed discussion of

Zr02/Ce02 materials is outlined in the manuscript of a paper ( Chemical Aspects

of Materials Containing Cerium Oxide ) to be presented at a meeting on ThermoPhysical Properties to be held in Moscow in May 1976.

3. Electrical Conductivity (A. J. Armstrong)

Most electrode sections were either cracked or bloated to such an extent

that very few samples could be cut that were large and strong enough for

electrical measurements- Only 5 anode sections could be used for electrical

experiments. Measurements were made on slices 2102 (88Zr02-12Y20-j ) and

2205 (82Zr02-18Ce02) - The conductivities of these materials after the

test differed surprisingly little (5-10%) from the results obtained on the

virgin samples.

4. .Radiographs of Electrodes and Lead-outs (E. N. Farabaugh)

In contact radiography, the image contrast is due entirely to the variation

in x-ray absorption from point-to-point in the specimen. Distinguishing

specific features in the radiograph depends on the relative contrast and thus

on tHe differential absorption of x-rays in materials of differing density

and atomic constitution. This method of characterization requires only an

x-ray source and an emulsion to record the image.

The accompanying radiographs (Figure 3) were taken using an uncolliminated

be.am of x-rays from a tungsten rotating anode operating at 200 kv. This

high voltage source allowed the viewing of the thick anode and cathode

electrodes in transmission. The images were recorded directly on type 57

Polaroid film. The specimen- to- source and specimen-to-film distances were

20 cm and 2 cm respectively. In the radiographs, the exact shape of the

electrode is not always recorded. This is due to the geometry of the

experimental arrangement and the size of the x-ray source. However, the

important features, the breaks in the platinum leads, are always easily

detected.

In the series of radiographs of the anode side, breaks are seen in

A2-7, A2-8, A4-15 and A5-17 . (See Figure 3 ). These electrodes were noted

as opening up during the test run. On the cathode side, Cl-1, Cl-4, C3-9,

C4-15 and C4-16 are seen to be discontinuous. Of the cathode elements, Cl-1

and cri-15 were noted to have opened during the test. The anode-cathode

pair 15 also showed open circuit during operation. Apparently, discontinuities

occurred in both the anode and the cathode Pt-wires. C3-9 behaved erratically,

but never opened up completely during the test. Cl-4 served as a position of

an optical sight port; it shows no lead in the radiograph.

The overall conclusions are:

I. The leads melted due to excessive Joule heating and were not

chemically attacked.

II. The leads should be of sufficient size to carry the current, be

thoroughly embedded in the cement, and be securely attached to

the ceramic tiibe (to prevent damage by twisting) .

Table I

U-02, Phase I, Pcst-Tesc

POST-TEST PHASE COl-IPOSITION (X-RAY ANALYSIS) FOR CATHODE MATERIALS

Initial Bulk

cposiclon (nol

Relative Position

of Material Analyzed

Results

Phases Present and Coinmentsi'

Zr02:18CeO2

1205

;,en Circuit

f?Zr02:18CeO2

il206

..25A/ce2

)"'Zr02:50CeO2

'i312

3pen Circuit

50ZrO2:5OCcO2

f;3C7

25A/cn2

:5Zr02:75CeO2

1414 f5A/cm2

':5Zr02:75CeO2

I.25A/cm2

upper 1-2 mm white naterial

white surface

yellow interior zone

yellow, olive-green bottom

white zone, top

yellowish interior zone plus

small portion of white zone yellowish interior zone plus

yellow-green bottom zone yellow-green bottom zone

yellow interior

yellow-green bottom

top white zone + yellow zone

yellowish interior

yellowish-green bottom

white (top) zone

yellow-brown bottom

dark yellow bottom

white (top) zone white + yellowish zones yellow-green bottom

Nearly pure mono Zr02 + f.c.c. solid-solution +

cubic and/or tetragonal solid-solution

Mono Zr02 + poorly crystalline f.c.c. and/or tet

sol id- solutions

Mono Zr02 (strong) + (Ci), (strong), + a trace

phase (P)

Mono Zr02 + (?)-Phase (very strong)

Mono Zr02 + two or more f.c.c. solid-solution phases

Mono Zr02 + strong (Ci)-phase, a^=5.26A

Mono Zr02 + (Ci)-phase + (P)-phase

(P)-phase strong + Mono Zr02 + (Ci)

Almost pure (Ci)-phase £=5.26A + trace mono Zr02

(Ci)-phase strong + additional cuaic phases with

similar but slightly larger a_ + trace (P)-phase

(Ci)-phase, + cubic solid-solution, + trace of a third cubic phase

Nearly pure (Ci) -phase, (a=5.26A) + traces of mono

Zr02 and a second cubic phase

(P)-phase (very strong) + (C2) £=5 . 38A phase

(strong) + small amount (Ci)-phase

Two f.c.c. solid-solutions. Major phase, a=5.35A;

minor phase has larger a^

Two f.c.c. phases. Major phase a^5.38A, nearly

that for pure Ce02 (a^5.42) (P)-phase also present

At least three cubic phases. Major phase a;=5.34A

f.c.c. a'>'5.34-5.35A + f.c.c. with slightly smaller a_

At least three cubic phases

At least three cubic phases + (P)-phase. Major

phase has a;^5.38A (similar to pure Ce02)

(Ci)-ph4»e refers to a pyrochlore type phase ;; (P)-phase is (K, Ce, Zr-oxide) perovskite; (C2) phase

•l«lUt to pure Ce02 : f.c.c. - face center cubic (Fluorite) ; Tet - Tetragonal; Mono - Monoclinic.

80TE: CotEpare results (phase assemblages and cell dimensions) with that given in Table 3 to assess

change In starting materials. Color changes are indicative of chemical changes.

TABLE I (continued)

Itlal Bulk

jBiposltlon (mol %)

,CeO2:20ZrO2:

^ 520

ipen Circuit

I

j ,Ce02:20ZrO2:

1517

.25A/cm2

iZr02:12Y203

1101

,25A/cm2

^Zr02:12Y203

il03

5A/cm2

Relative Position

of Material Analyzed

yellow top zone

green bottom

yellow zone upper portion

yellow zone lower portion

bottom zone

bottom, sample near MgO

insulator

bottom, sample next to MgO

insulator

white surface

Results

Phases Present and Comments

Two dominant cubic phases {^5. 26k, a'^S.ZSA)

Nearly single-phase f.c.c. £=5.35A

At least two f.c.c. phases. Major phase, a^=5.37A,

minor phase has smaller a'^5.28A

At least three, possibly four cubic phases

f.c.c. £-5.154A, + K2C03«nH20

f.c.c. + K2C03-nH20

Two f.c.c. phases plus small amount of mono Zr02

Well crystallized f.c.c. phases a^S.lASA

1

TABLE 11^

Anode Electrode X-ray Results

1.. Composition ~ 88% Zr02, 12% Y2O3

o Original - Very well-crystallized f.c.c, a=5.151A

o Post Proof Test - Well-crystallized f.c.c, a=5.136A.

Very slight suggestion of line splitting which could

indicate lower symmetry or multiple phases.

o Post Phase I Test Samples - bulk material f.c.c, a=5.15A

o Surface crystallinity good, f.c.c, a'^>5.14A.

Intensity varies from original. Line

sharpness varies to suggest splitting in

some directions. ' •

o Surface crystallinity good, f.c.c, a'>>5.14A

Oriented intensitites . Some peaks show

distinct line splitting.

Similar to 2101. Oriented intensities and

probable line splitting. Approximately

o f.c.c. , a=5.14A.

o Approximately f.c.c, a'^'S.lAA. Oriented

intensities and some distinct line splitting

Small amount of spinel visible.

2. Composition — 82% 7x0 7, 18% CeO 2

o Original - Almost all tetragonal, a=5.15, c=5.24A

o Post Proof Test - Almost all f.c.c, a=5.11A.. (Surface)

Post Phase I Test Samples - Crystallinity generally poor although some

of the poor resolution may be due to uneven surfaces.

2205-2208 All surface patterns show primarily a cubic (or very

o nearly cubic) phase with a'\>5.1A. They also show monoclinic Zr0 2 and spinel (probably MgAl20^)

2101

2102

2103

2104

Composition ~ 50% Zr02, 50% Ce02

o Original - Primarily tetragonal Ca=5.26, c=5.30A) plus a small

o amount of a cubic phase (a=5. 15A) or tetragonal

o (a=5.15, c=5.2AA) and a trace of a probably cubic

o phase (a=5 . 34A)

o Post Proof Test - Very poor pattern which is primarily f.c.c, a=5.12A.

Post Phase I Test Samples - very poor condition. Only the 2312 sample

could be used to obtain a surface pattern. All other patterns

were obtained by grinding samples from the top and bottom portions

of the electrodes.

o 2309 Top portion is mostly cubic (or nearly cubic) a=5.26A.

It also contains small amounts of approximately cubic

o o cells with cell edges of 5.21A and 5.16A plus a small

amount of monoclinic Zr02.

Bottom portion has three phases present which are

o similar to the original material: cubic, a=5.33A;

o o tetragonal a=5.27, c=5.31A; tetragonal a=5.15, c=5.24A.

2310 Top portion is mostly nearly cubic (probably tetragonal

o a=5.26, c=5.29A) plus small amounts of material with

o o smaller cells ('^^5.21A, '^^5.13A).

o Bottom portion is mostly cubic (a=5.34A) plus some

o tetragonal (a=5.15, c=5.24A).

o 2311 Top portion is almost all tetragonal (a=5.26, c=5.30A)

plus a trace of a smaller tetragonal phase (a=5.15,

0=5.24!). .

o

. - Bottom portion is mostly cubic (a=5.34A) plus some

o tetragonal (a=5.15, c=5.24A).

17

2312 The surface is mostly two phases: - cubic (a=5.14A) and

o tetragonal (a=5.l8, c=5.26A). Small amounts of monoclinic Zr02 and spinel are also visible.

The bulk surface material is primarily cubic (or nearly

o so) with a 'X'5.27A. It also contains smaller amounts

of phases with smaller cells with cell edges 'V'5.21,

5.16, 5.14A.

A. Composition — 25% Zr0 2, 75% Ce0 2

o Original - Well crystallized f.c.c, a=5.3A2A

Post Proof Test - Very poorly crystallized mixture of at least three phases

of cubic or nearly cubic materials with cells %5.33, 5.26,

o 5.19A (surface).

Post Phase I Test Samples were considerably fragmented. Surface patterns

could be obtained only on the 2413 and 2416 electrodes.

o The surface is mostly a cubic phase (a=5.14A). There

is also some spinel visible plus traces of monoclinic

o Zr02 and a possible cubic (a=5.25A) phase.

Could not get a useable surface pattern. The ground

upper portion is mostly the same as the original

o material (cubic, a=5.34A). It also contains small

amounts of two other phases: tetragonal, a=5.23,

o o c=5.29A; probably cubic 3^5. 16A.

No surface pattern. A loose piece which was mostly

surface material was ground for x-ray examination.

o It was mostly cubic with a'\.5.13A plus a small

o o amount of two probably cubic phases, a=5.22A, a -5.19A.

Surface pattern is predominantly cubic (or nearly so)

o with a'v.5.2A. Small amounts of a minor cubic component

o (a-^vS.BA) and spinel are visible.

2413

2414

2A15

2A16

Composition -- 20% Zr02 , 78% Ce02 , 2% Ta205

Original - Consists of a major component (well crystallized f.c.c,

o a=5.35A) and a minor component (not as well crystallized

o f.c.c. , a -5. 37A)

Post Proof Test - Pattern is nearly cubic, but the crystallinity is only

fair. Slight^ splitting of high angle lines suggest that

o it may be tetragonal with a=c=5.3AA or two cubics with

a=5.3AA and a '=5.351.

Post Phase I Test Samples - All samples have a thin white coating

(presumably due to loss of Ta) and are green like the original

beneath this surface coating. The bulk green material is the well

o crystallized f.c.c. phase (a=5.35A).

The white coating, when chipped off and ground, is primarily the

o f.c.c. phase (a=5.35A) but, also, contains at least two other

o o phases with smaller cubic or nearly cubic cells (a''^^5.2A, a"'\'5.3A)

which are presumably richer in Zr02.

The surface patterns of all these samples show primarily a poorly

o crystallized pattern of the 5.2A material plus smaller amounts of

o the 5.3A material. Additionally, the surfaces showed:

2517 Traces of spinel and an unidentified material.

2518 Traces of monoclinic Zr02 and an unidentified

material.

2519 No additional phases.

2520 Trace of spinel.

1 o

n

A3

12

13 14

A4

Figure 3. Radiographs of Anode Submodules 1 to 5.

Task I. OPERATIONAL DESIGN PROPERTIES

a. VISCOSITY OF COAL SLAGS (W. Capps and D. A. Kauffman)

Reduction-Oxidation Effects on Viscosity

An experiment was conducted to detenrine if there is a change in

viscosity with a change in the oxidation state of iron in coal slag melts.

A slag composition having a moderately high iron content was selected

which had been melted previously. K385, containing 40% by weight of SiO^,

15% Al 0 , 20% Fe 0 , 17% CaO , and 8% MgO, was duplicated and numbered

K554.

It was assumed that the only ions in this synthetic slag that could

be shifted easily from one valence state to another were the iron ions.

Presumably, by changing the availability of oxygen to the melt, at least

some of the iron could be shifted from the divalent state to the trivalent

state and back again. It is conceivable that this could affect the viscosity

of the slag. If so, the extent of this effect should be known for purposes

of slag-wall thickness estimates in operating coal-fired MHD systems.

An alumina tube was used to bubble gases through the melt before and

during viscosity determinations. The solid curve in Fig. 4 shows the

viscosity-temperature relationship of K385. This was reported earlier.

The melt v/as exposed to the air in the furnace atmosphere just as all the

other melts were that have been described in preceding reports. Figure 4

also shows data points for K554 with and without bubbling. The data points

are numbered in chronological sequence. It is apparent that the two

melts, presumably identical in composition, showed similar but not

identical viscosity behavior. Points 1 and 2, at about 1530 C, show the

negligible effect of bubbling with 5% in argon. Of course, the crucible

was open to the air in the furnace and probably there was an abundant supply

of oxygen to diffuse into the glass to counter the reducing effect that the

hydrogen may have had. This comparison also holds at 1370 C as shown by

the points 3 and 4 which are identical.

When oxygen was bubbled through the melt, however, an increase in

viscosity occurred at once. The effect was not large but quite visible.

After about 3 1/2 hours, the viscosity had increased by more than 7%.

After bi.±)bling was terminated, the viscosity dropped again but not down to

the level before the o>ygen bubbling began.

This experiment was preliminary in nature. The conclusions must

necessarily be tentative. One can conclude that the viscosity can be

altered by bubbling 0^ through the melt. If a reducing atmosphere is

desired, it may be necessary to go to greater pains to keep oxygen from

the melt surface. It is possible that a covered crucible would permit

a greater fluxing effect and be more effective in blocking the counter

movement of oxygen back into the molten slag.

This effect will be explored further in the future.

Rebuilding NBS Sl^g Preparation Facility

After three years the heavy load on the melt furnaces in the Inorganic

Glass Section imposed by the MHD viscosity program, along with heavy demands

from other programs in progress, finally caused sufficient deterioration

to necessitate rebuilding them.

Little melting of new slag compositions was possible during this

reporting period.

Accomplishments in refurbishing the melt facilities are as follows:

(1) Two melt furnaces were completely rebuilt with new bricks, a new electrode configuration was designed, and silicon carbide resistance

elements v;ere installed.

(2) Ancient and overloaded transformers were replaced by new transformers each having 5 ten-volt increments in their secondary windings.

(3) Phase angle fired, silicon controlled rectifiers, controlled

by thermocouple-fed digital proportional controllers, were installed for

melt and annealing furnaces.

(4) Digital temperature readout was also provided for melting and

annealing furnaces.

This installation is now complete and in op'eration and provides a

completely modern, stepless control of melting and annealing temperatures.

High Tempnrature Viscometer ^

The high temperai-.ure adaptation of tha present viscometer is still incomplete. The iridium spindle and crucible have been received. The alumina

extension of the iridium spindle has not yet been received. Adaptation to

the high temperature graphite furnace is not yet complete.

Future Plans

1. An attempt will be made to remelt the Fluidyne Air Heater

flyash in an alumina crucible and an alumina spindle will be used to

measure the viscosity. This flyash would not make a homogeneous melt

and caused disasterous disintegration of the platinum crucible, as

reported previously.

2. Bubbling experiments will be resumed in alumina instead of

in platinum to permit more strongly reducing atmospheres to be used.

3. High temperature modification of the viscometer will be

pursued.

4. An effort will be made to modify the equations developed

earlier to include the effect of potassium oxide on the viscosity.

5. A flyash and a bottom ash from Rosebud Coal Se-im, as provided

by Montana State University, will be melted and their viscosities

determined.

23

Viscosity dog^o P^^^^^^

U1

U1

o —

M 13' 0 TO i-h rt i-h 0 cn fD 1—

O

o 1 H- rt M 3

0 ?^ M- 0

yQ Hi

cn &> o

ft) H* •—

cn o

0 u> OJ X

3 rt o

to o

n • t}

• o Qj

3 H- ^ rt 0 0 H01 M 0

H- H- t5 rt a cn H-o o o

c

^ < rt C fD

fl- s: <

rt

(D cnn

rt 0

3 1^ cnH- M rt

rt ^

iQ

• M O

(D Ml

(0

cn

(P u>

CD rt

tn 3'

• (Drt ^ H- O n

H- o 3 o

(t) 01 H M

(DOi cn

vQ

O

o

U)

o

o

(13 3

fl

rt

^^

n

o

o

O

o

O

O

o

o

CO cn

CD cn

cn j;:^

VOCD^CTiCnoriCiJNJM ^ ^*^ ^

O O O O

Cd 3 CO DO & ° cr trcrcrcrcrcrcrcrD'

crcrcrcrD cr:3 crn

I-" 1-' M M U3 h-

(j. p. p. p. p. p

iq (jq ua u3 h- u3 h- lij pft rt ft

I I I I tr I 3" I :3'

2

O

n ^ > c«

OJ fD t-> HC2CO NJ O O O NJ

CO

h-' cncn

CD NJ M

-J O cn O

0) 0) O 0) o Dj cn OJ U1

H- H- < o'P H-

^(

er a:

»^

OJ 01 OJ OJKJ oiro

rt H- rt rt g 3 vn 3 g (-• 3

o O 3" O o O

cn cn rt cn cn cn n3 ^ 'a OJ 0)

irr :r 13- 3" f-( fD CD fD m (D M O ^t O

CD CD fD fD 3 (D 3

24

b. Electrical Conductivity (W- R. Hosier, A. J. Armstrong, H. P. R. Frederikse)

Spinels

Solid solutions of Mg-chromite or Mg-aluminate spinels and magnetite

(Fe^O^) appear to be promising materials for MHD electrodes. During this

reporting period the conductivity of more than a dozen spinels of various

Mg-Al-Cr-Fe ratios were investigated. The sources and compositions of

these materials are listed in Table III. Of the group of A. P. Green spinels

only the "ISO-C" has sufficient conductivity (a>10~2 ohm~icm-l at 1000 K)

in the second batch all 5 materials listed are acceptable from this pointof-view. The chromite of the single phase spinels (second group) have somewhat

higher conductivities than the aluminates. However, the aluminates are

preferable because of their larger corrosion resistance with respect to

potassium seed (see discussion under Task J) . Fe-Co-spinel appears to have

good characteristics as a lead-out material.

A paper discussing "Spinels for MHD-electrodes" will be presented at

the 15th Symposium on Engineering Aspects of MHD (May 24-26, 1975, Philadelphia)

Several other spinels of similar composition were sent to NBS by

Dr. Rossing (Westinghouse Research Labs) . Two of these were Corhart

materia].s labeled CClOO and CC104 respectively. Their nominal compositions

were (in wt. percent) : (CClOO) "^10 MgO-10Al2O3-10 Fe2O3-70 Cr203 and

{CC104) '^^56 MgO-9Al2O3-10Fe2O3-20Cr2O3 . A third sample prepared by

Westinghouse) (SW) was mainly Mg-chromite as indicated by chemical analysis:

10.3 Mg-50Cr-<0. 025Fe-<0.15 Al (wt. percent).

The electrical conductivities of these three materials are shown in

Figure 5. The level of conduction in samples CC104 and SW is quite high.

The Corhart specimens, however, are very porous and inhomogeneous and

hence no further studies of these materials were made.

Graded Cermet (UCAR)

The apparatus for measuring the electrical conduction under large

temperature gradient conditions has been used to explore the conductivity

of a graded ceramic/metal sample furnished by Union Carbide (Parma, OH).

This material is used for the wall of jet engines; it is about 1/4" thick

and consists of Y-stabilized zirconia graded with Ni and Cr powder from the

center of the sample down to nichrome metal at the bottom. Surface

temperatures of up to 1800 °C were established by means of laser heating,

while the nichrome metal soldered to a water-cooled copper block was held

close to room temperature. A thermocouple at the interface between the

graded oxide + Ni + Cr and the stabilized zirconia indicated a temperature

of 585 °C. Under these conditions the total resistance of the sandwich

was 65 Kfi. The interface temperature could be raised 200-250 °C to about

800 °C without danger of Ni or Cr oxidation. However, even at that level

the zirconia resistan::e still is much too higli to permit current

transport of the order of 1 to 10 Amp.

25

I»bl« 1 11 .Composition of Spinel Samples Stiidied

MgO ^2°3 '^)°2 ^^^3°4^ ^°^°4 ^^°2 porosity(\)

A

/ u 117 1 7 n e -> 3 . ^

1 A 7 7A n / . J aU. 7n X1 ^w. 7n n

0.0 O.J

17 7 ^•i fi XO . ^ . \J XX17 . 7/ xw • *

17 7 5^ J / • J

J . o 1 J * J J . XM 4.0

. sp. iin/o; x?.a~ 1Q nx> xo7n . A

XInU . AH B-3201D (w/o) 27.0 38.7 10.4 22.8 0.82 0.27

«p. (m/o) 50+ 25.5 10.25 14.2

T1 7 10 7 18.3 0.96 0.31

24.0 12.5 11.5

•Chromitc #2 (n/o) 42.1 42.1 15.8

(44.4 44.4 11.1)

•Chraaite «6 (m/o) 36.4 36.4 27.2'

(40.0 ^0.0 20.0)

•Aluminate #3 (m/o) 42.1 42.1 15.8

(44.4 44.4 11.1)

•Altsainate #5 (m/o) 40.0 40.0 20.0

(42.8 42.8 14.3)

*Pe-Co spinel (m/o) 70.8 29.2

^- These sanples were prepared by the A. P. Green Refractories Company. The chenical concosition in weigh

percent is given on the isz line of each senple. These specimens wero not sinole phase, cut a mixture

of spinel and excess (cubic ) .".~0 or excess Cr_Ot { sar.pie i50-C). The p;econd line gives the composition

of spinels (s?.

)

in raole percent (+ indicates larqe anounts of excess M9O or Cr.,0^) . These samples wer e prepared in the Solid State Chemi stry Section of N5S. All sanples were single phase

•plnel. The face that the noie percentage of MgO is smaller tnan 50 indicates that these spinels are partially "inversed".

I



c. Vaporization Studies (E. R. Plante)

The vaporization of K^gx + 02(g) from two synthetic slags, one containing

a high CaO content (15 wt,%) the other high in AI2O-J content (20 wt.%) , was

measured using the mass spectrometer - Knudsen effusion method. Pertinent

data, results and conclusions will be given in a paper by Cook, Plante,

Negas, Roth and Olson to be presented at the 15th Symposium on Engineering

Aspects of MHD (May 24-26, 1976, Philadelphia). Significantly, both

slag samples show higher K pressures than would be expected for "slags"

containing only K20/Si02. This implies that simple seed-slag calculations

using pure K20/Si02 solutions to model -t-.he interaction will predict concentrations of K2O in the slag solution several times greater than in reality.

The high potassiiam pressures over the CaO-rich slag appear to support the

concept that high calcia content in slag will increase the K2O activity by

reducing the SiO^ activity (hence, less absorption of seed)

Future Work: Measurements of K pressures over slags of various compositions

will be made in an attempt to determine the effects of various components on

the activity of K2O. Vaporization measurements on a pure K20-Si02 solution

will be attempted using smaller orifice size to determine if saturation

effects were important in previous measurements.

28

TASK J. CORROSION PROCESSES, SEED-SLAG INTERACTION, DIFFUSION

a. Spinels (T. Negas, W. R. Hosier)

In the previous Cu^^terly Report the spinel MgAl204 (and MgO plus

MgAl204) was shown to have promising resistance to attack by seed. This

was based on equilibrium data which demonstrate that MgAl204 does react

with K2CO3 to yield MgAl204 + MgO + KAIO2 but only very sluggishly.

Potassium 6-alumina cannot be a reaction product unless the ceramic

consists of spinel plus ^12*^3 ^^'^ vaporizes preferentially from

MgAl^O^ to drive the bulk composition toward alumina-rich compositions.

The spinel MgCr204 also has been tested with K2CO2 . The composition

MgCr204:2K20 (2K2CO3) was formulated and fired at 900 °C in air. Within

one hour a liquid phase developed indicating that phase relations were

migrating into the MgO-Cr203-K20-oxygen quaternary. Continued reaction

at 900 °C, 16 hours, yields

MgCr20^ + 2K^0 + 1 1/2 MgO + 2K2CrO^ (1) .

The corrosion product K2CrO^ melts at 971 °C and, hence, liquid formation at

low temperatures is expected when MgCr^O^ reacts with seed. It should be

noted that reaction (1) proceeds to the right only when the oxygen activity

is high. Meadowcroft^ tested MgCr20^ by exposing the material to a simulated,

seeded, MHD environment at 1620 K, 100 hr . Results indicated that resistance

to seed is extremely poor, the material dissociating to leave only MgO.

Some of the spinel samples listed below have been exposed to K2CO3

in a covered Pt-crucible for periods of 4 hours at 800, 1000 and 1200 °C.

In each case the chromite-spinels were attacked by the potassium forming

bright yellow K-chromate. The Mg-Fe-aluminate spinels showed no visible

signs of any chemical reaction. Some weight gain occurred due to seed

penetration into the pores of the samples, followed by further weight increase

when the K^CO^ hydra ted during storage in the laboratory.

Six spinels were prepared based on the chemical considerations outlined

in the December 1975 Quarterly Report. These materials are single phase

spinel solid solutions prepared at elevated temperatures from the following

initial bulk compositions:

1) 2MgCr20^:l/2Fe^O^

2) 2MgCr20^:Fe^O^

3) MgAl^O^rFe^O^

4) 2MgAl20^:l/2Fe^O^

5) 3MgAl20^:Fe30^

6) 70(wt%)Fe20^:30Co3O^

The electrical conductivities of all of these materials have been measured

(see Task I) • Composition (5) appears attractive from the standpoint of a)

suitable conductivity, b) low iron oxide content and c) high melting point

(>1750 °C, in air) . Composition (6) offers resistance to oxidation at low

29

temperatures and high oxygen potential but is less refractory. Hence, it

could be used within the lower temperature portions of an electrode module

graded with, for example, composition (5).

The compatibility of LaCr02 , another lead-out candidate, with MgCr20^

spinel was demonstrated by firing the composition LaCrO :MgCr^O^ at 1400 °C

and 1500 °C, 3 days. No reaction was^observed in accord with'the results

of Dr. B. Rossing, Westinghouse Corp. Additionally, MgO (insulator) does

not react with LaCrO^ as evidenced by firing the composition, MgO:2LaCr03,

1500 °C, 5 days. No reaction was observed. These data indicate that an

electrode module composed of MgO (insulator) - LaCr03 (lead-out) - MgCr20^

(hicher temperature portion of an electrode) represents a compatible phase

assemblage. It appears likely that this relationship should hold for spinels

other than MgCr20^.

References

1. D. B. Meadowcroft, Energy Conversion 8_, 185 (1968) .

2. Dr. B. Rossing. personal communication.

b. SEM and EDX Characterization of Corhart X-317 Refractory Ceramics

(spinels) Tested at FluiDyne Eng. Corp . (E. N. Farabaugh)

Preliminary characterization of Corhart X-317, 50 MgO + 50 MgAl^O^ (wt. percent) refractory ceramic body, has been undertaken to determine elemental

distribution near the slag-refractory interface and also in the bulk of

the. refractory.

Figure 6a is a 16X SEM micrograph of a section of specimen FN-27,

material #39, lot Corhart X-317, and test No. 53, 1800 K for 30 hours.

The area on the left of the micrograph that is porous looking is the

edge of the specimen that had direct exposure to the slag. The boundary

of the slag layer is defined by the boundary between the porous material

(slag) and the more dense appearing material (refractory)

Figure 6b is a Si map of the same area at the same magnification. The

silicon rich slag layer is sharply outlined at this magnification. There is

evidence of some Si in the refractory also. Ca and Mg maps are shown in

Figures 5c and 6d, Again a rather sharp boundary between the slag and. the

refractory is revealed. Maps for K, Fe, and Al were also taken. These

indicated K pres'3nt in both slag and refractory with higher concentration in

the slag. Fe was also present in the slag layer. Al was distributed almost

uniformly across the slag and the refracto:i."y . Examination at higher magnification, to be done at a later time, will determine if any diffusion of the

slag constituents into the refractory has occurred.

Figure 7a shows a SEM micrograph of one of the edges of the specimen

which was not directly exposed to the slag. Figure 7b is a Si map of the

same area at the same magnification. Comparing Fig. 6b and Fig. 7b it is

noted that the uniform Si distribution in Fig. 6b is due to a uniformly

thick slag layer aad the uneven thickness of Si in Fig. ^b is due to an

uneven slag layer. The rounding of the corner of the specimen (seen in

Figs. 6b, 6c and 6d) must be due to erosion of the specimen and not due to

penetration of the Si and Ca into the refractory to give the appearance of

a rounded corner. The Mg map also indicates that some of the refractory

Fig. 6a . 16 X SEM micrograph #FN-27

Corhart X-317.

Fig. 6b iji Map of the Same Area

Showing the Slag Layer

Refractory Boundary.

Fig, 6c. ca Map of the Same Area Also

Showing the Slag Layer

Refractory Boundary.

Fig. 6d . Mg Map of the Same Area

Showing the Same Slag Laye:

Refractory Boundary.

Fig. 71: . Si Map of the Same Area Showing the Slag

Layer Refractory Boundary.

body has been removed at the corner, (It is assumed that the specimen was

fabricated with sharp corners)

Figure 8a (p. 34) is a 1600X SEM micrograph taken well away from the slag

layer and in the interior of the specimen. Figure 8b (p. 34) is an EDX spectrum

of the same area. The predominate lines indicate the prensence of Mg, Al

,

(the two strongest peaks) Ca , and quite small amounts of Cl and Si. It is

felt that the Ca, Cl, and Si detected must be present as additives to the

refractory and not due to any diffused material.

Figure 9a (p. 34) is a 1600X SEM micrograph taken in the slag layer. There

isn't a great difference in the appearance of the slag surface and the

refractory surface. It is possible that polishing and etching these surfaces

would bring out different surface features. Figure 9b (p. 34) is an EDX

spectrum of the same area. Note now we have three strong peaks, Mg , Al

,

and Si and many smaller peaks. The other elements in the slag are S, Cl

,

K, Ca, Ti, Cr and Fe.

This work will continue and data will be taken to determine if any

elemental concentration gradients exist in the slag or the refractory.

Areas surrounding cracks and large pores in the refractory will be examined

to determine if they have provided an easy path for elemental diffusion.

c. Diffusion (E. N. Farabaugh, J. R. Manning)

Work has continued on the study of the penetration of K into materials

used in the MI^D process. Previously, a diffusion couple of ceramic AI2O2

embedded in B-AI2O3 was heated at 1400 °C for 92 hours and the results

reported. Since the last reporting period, two more couples have been made.

Couple #2 was again ceramic AI2O2 embedded in 6-A1202- This couple was

heated at 1400 °C for 24 hours. Couple #3 was ceramic MgO (85% dense) embedded

in B-AI2O3. This couple was also heated at 1400 °C for 24 hours.

With this treating time, it was expected that the K would not penetrate

as deeply into the AI2O2 as it did for the 92 hour run, and that the concentration profiles from opposite edges would not overlap. However, K maps of

the specimen that were taken indicate vanishingly small K concentrations.

Figure 10 (p. 35) shows a 18X SEM secondary electron image micrograph of an

edge of couple #2. Fiaure 11 (p. 35) is a K map of the same area. There is

little concentration of K shown. A point-by-point analysis did show some K

present in a narrow region near the surface. An analysis done at higher

magnification will be done in an attempt to pick up any K gradient. More

AI2O2 couples will be made at this temperature and time- Concentration profiles

will be determined to verify that the K concentrations are as low as they appear

in thxs experiment.

Some time has been spent preparing our laboratories for the arrival

of the new SEM and EDX and accessories. With this new equipment on line,

it is anticipated that problems encountered in doing EDX analysis with

the older available equipment will be eliminated.

33

Fig . 8a. 1600X SEM Micrograph Taken

Well Away from the Slag

Refractory Boundary.

(Unpolished Surface)

Fig. 8b. r^DX Spectrum of the Same Area

Shpwn in the Micrograph. Larg

Peaks are due to Mg and Al

Fig. 9a , 1600X SEM Micrograph Taken Fig. 9b. EDX Spectrum of the Same Area

in the Slag Layer. (Unpolished Shown in the Micrograph.

Surface). Large Peaks are due to Mg

,

Al and Si.

Fiqure 10 , isx Secondary Electron Image Micrograph of an Edge

of K-dif fused Al 0 Specimen.

Figure 11. K Map of the Same Area at the Same Magnification.

As another part of the present diffusion work, the kinetic diffusion

theory necessary to predict how diffusion interactions can influence

degradation of MHD materials is being developed. When ions diffuse

through regions of material having regular crystal structure, without

cracks, pores or similar gross imperfections, kinetic interactions

between the different metal ion fluxes taking place on the same sublattice occur. This especially happens when electric fields and

temperature gradients bias these fluxes, as customarily is the case in

MHD systems. To derive equations describing these interactions, the

relation between the non-random diffusion that results from temperature

gradients and the non-random diffusion known to arise from correlated

ion jumps in isothermal systems is being investigated. It is found

that similar parameters enter into both types of non-random calculations.

d. Seed/Slag Interaction (L. Cook)

The most important factors in determining seed/slag interaction are

1) partial pressure of potassium in the channel, Pj^; 2) temperature at

which the interaction takes place; 3) the bulk chemical composition of

the unseeded coal ash. Factors 1 and 2 can be calculated, estim.ated or

observed. The comjjosition of coal slag, a chemically complex substance,

requires systematic analysis before generalizations suitable for the purposes at hand can be made. As an initial step in this direction, consider

the analyses given by Selvig and Gibson (Bureau of Mines, Bull. 567 (1956), Table 2),

which show the variability of coal ash in a typical bed of Pennsylvania coal. Recalculated, the analyses plot as shown in Figure 12 below, for an assumed

Fe'^-^/Fe^ = 0.20. Si/Al+Fe ratio is nearly constant at '^1.5. Inspection

of other analyses listed in U.S. Bureau of Mines Bulletin 567 shows that

the above range of analyses may be representative. If so, then the studies

of seed/slag interaction should concentrate on compositions close to the

range shown in Figure l2 CaO-Al202-Si02 may thus prove useful as a modal

system with which to consider the interaction of seed and slag.

As an example of this approach, consider the application of results of

quench experiments on K20-CaO-Al202-SiO^ mixtures presented in the previous

quarterly report. These results may bemused to construct the schematic

phase diagram shown in Figure 13 below. The effect of adding CaO to compositions rich in KAlSiO^, a compound thought to be responsible for tying

up much of the unrecoverable in MHD slags, is clear. If CaO is added

so that the bulk composition moves from three-phase filed A through B to

C, 3-Y_20 will increase progressively, assuming that a slight excess of

component ^-12^3 present so that Sj-^q is buffered. If a^^ q is increased

by addition of CaO, then Pj^ must be increased, as well, meaning that for

a given Al/Si ratio addition of CaO is likelv to result in decreased K2O

absorption by the coal slag. Whether this is true in general, must of

course be tested by experiments. It is likely that FeO and MgO have similar

effects upon aj^ q since they are chemically similar, and because all are

capable of substitution for each other to a large degree in silicate phases.

Further studies of seed/slag interaction will concentrate on determining,

for a reasonable range of idealized coal slag compositions, the equilibrium

phase assemblages that result when K2O is added. This equilibrium data will

allow more detailed prediction of seed/slag interaction under channel conditions.

Preliminary indications are that the phase equilibrium data, when analyzed

and presented in terms of the chemical potential of K„0, V^aO' 8-^^^ ^ clearer

visualization of seed/slag interaction than do conventional diagrams. Given

In this way, the data also allow direct correlation with vaporization data.

C«^l,0.

Figure 12 Recalculated analyses of coal ash fron

Pennsylvania coal (dots), shcuTi in relation to

Iconpounds in the system CaO-Al203-Si02 (ooen

i circles). Dashed line shows Si/ (Al + Fe* ) = 1-5.,

CaO

Figure 13 Effect of adding CaO to compositior'!

llch in KAlSiO* and near the ternary plane KAlOjSlOa-CaO.

Seed Regeneration (P. Skarstad)

Spent seed in the form of potassium sulfate can be separated from

the insoluble portion of coal ash by extraction with water. The sulfate

may then be removed from solution and replaced by an anion suitable for

recycling into the combustor. Sulfate can be precipitated with Group II

cations, the most readily available of which is calcium. Choice of anion

involves the following consideration:

1. The anion should not contain elements which form undesirable

combustion products, e.g. halogens, sulfur.

2. It should be inexpensive and readily available in large

quantities.

3. Both the calcium and potassium salts should, be much more soluble

in water than calcium sulfate (K^ = 2x10 ) . sp

A. Hydrolysis should occur to a small extent. In other words the

anion should be the conjugate base of a reasonably strong acid

consistent with requirements 1-3.

The anions of certain common industrial organic acids seem to fulfil

all four criteria. Formate appears to be the best of these based on

availability, solubility of the final potassium salt, process flexibility,

and recovery efficiency. Thus, a version of the "Formate Process", used

commercially for conversion of K^SO^ to K^CO could probably be adapted

to the requirements of I'lHD seed recovery. This has been pointed out

previously [1] .

The commercial "Formate Process" involves the following:

1. Generation of calcium formate from CO and CaCOH)^

2. Precipitation of calcium sulfate, leaving potassium formate

in solution

3. Oxidation of potassium formate to potassium carbonate

At the carbonate stage, sodium can be separated from potassium by

fractional crystallization. It might not be necessary to carry through the

oxidation to carbonate, however if potassium could be introduced to the combustor as the formate. This should be studied. Potassium formate is A6 wt%

the remainder is combustible. Potassium carbonate is 56 wt% K. Saturated

potassium formate solution (80°C) is 40 wt% K and 13 wt% H„0. The remainder

is combustible. K2C02-1.5H20 (100°C) is 47 wt% K and 16 wt% H^O.

If, after severax cycles, the sodium content of the regenerated seed

builds up to unacceptably high levels, the Formate Process could be carried

through to the carbonate, and the sodium carbonate could be separated by

fractional cryotallization.

Reference

1. AVCO Fuel Report to EPRI , MHD Power Generation EPRI 97, June 1975.

Task K. MATERIALS. TESTING AND COORDINATION (W. R. Hosier, H. P. R. Frederikse)

a. Plans for the U-02 Materials Test (Phase II)

Work for the implementation of Phase II of the US-USSR Cooperative

Materials Testing Program is being continued. This test is scheduled to

begin in mid-August 1976 and is to run for a minimum of 100 hours. Six

materials will be tested in the U-02 channel using a configuration similar

to that of the Phase I module. However, the electrode pitch and thermal

gradients will be altered to correspond more closely to the geometry and

heat fluxes of the channel being designed for the Soviet U-25 facility

s'window frame design, width per window "^1 cm). Other modifications will

be made, based on the experience gained in the Phase I test, especially

with respect to porosity and insulator geometry (to counteract seed

penetration and thermal shock)

The Phase Il-module is being designed and will be constructed by

the Westinghouse Research Laboratories. NBS is keeping close contact

with Dr. Rossing (Westinghouse) and with other contractors involved in

the U-02 testing program. Samples of spinels have been prepared and sent

to Westinghouse for possible use as Phase II electrodes. At the same time

several candidate materials have been received from other organizations

for the deteirmination of structural and physical properties.

A work plan similar to the one for Phase I will be drafted.

b. Plans for the U-02 Materials Test (Phase III) .

As mentioned under Task H the result of US-USSR discussions during

the February meeting of the joint Materials Working Group is that both

sides want to go ahead with the planning of a third materials test in

the U-02 MHD-facility . The U.S. side has suggested to make the major

goal of this experiment the testing of electrode and insulator materials

under maintenance (replenishment) conditions. Furthermore, it was

recommended that the maintenance materials will be Fe-Al-Silicate . More

detailed plans both from the American and from the Soviet side will be

submitted to the Steering Committee in June 1975 (Washington, D. C).

c. P.oat Test AnaJ.ysis of AVCO MHD Channel Electrodes

Eight electrode sections were obtained from the AVCO MHD channel subsequent

to the ICO hour power run completed in late February. Each of these sections

consists of two or more electrode elements. Anode and cathode sections were

taken from the upstream end of the channel where the "button" type construction

was used and from the center of the channel length where rail type construction

was used. A minimum of three slices from each section were cut perpendicular

to the electrode surface and parallel to the plasma flow. This allows a

cross sectional view of the electrode assembly with the slag in place

above the electrode. Unfortunately, during the section removal from the

channel, the slag chipped loose in some areas. As a result the cross

section shows only proximal slag. All cross section pieces were held in

place by epoxy pot.ting prior to the cutting process. The slag separation

is not a serious difficulty since the SEM scans and microprobe can traverse

this void area and pick-up the proximal slag without loss of information.

Each slice will be examined m detail m all areas. A general montage

of the electrode area will be made initially to identify those areas of

particular interest where information can be obtained to help ascertain

the electrode erosion and corrosion problem.

The initial questions for which answers are sought in this investigation

1. What causes the unacceptable high rate of metal loss at the anode?

2. What is the charge transport mechanism in the slag layer and at

the slag-metal interface on both anode and cathode?

3. What causes the erosion of copper on the upstream side of the

cathode electrode?

4. What causes a degradation of the cathode sidewall insulation?

Results : Slicing of the electrode sections is nearly complete. Several

sections have been repotted for SEM and microprobe work and some information

has been obtained concerning the chemical attack on the cathode. Erosion and

corrosion effects on the cathode side are slight but still are readily

observable. Detailed information about electrode effects - in particular,

results that will help to answer the four questions stated above - will

be forwarded to ERDA and AVCO when the analysis is completed.

Summary : Analysis of AVCO MHD channel electrodes has been started. The

investigation will concentrate on the causes of the erosion and corrosion

experienced during the recent 100 hour test. Final results and conclusions

will be presented in a later quarterly report.

are:

Task L. ASSESSMENT OF STEAM PLANT COMPONENTS (J. R. Cuthill)

During this reporting period the first stage of Task L has been

completed. Our goal, which is to deterTnine performance requirements, has

been met in so far as projected values of gas temperatures, mass flow,

and gas composition, and steam temperature and pressures have been forthcoming from computer model calculations. These "performance requirements"

will be continually revised in forthcoming reports as ref±ned and additional

data become' available. For example. Figures 11' and ij are updates of

Figures 23 and 24 in the previous , quarterly report, by the addition of

design values of steam temperatures and pressures of 1000 °F and 3500 psi

steam leaving the Steam Super Heat, and 1000 °F, 650 psi steam leaving the

Steam Reheat, labeled "Steam No. 1" and "Steam No. 2", respectively. These

values were obtained from the STD Corporation. With a steam temperature

oc 1000 °F on the inside of the boiler tube, the outside wall temperature

of the tube (exposed to the flue gas) is estimated to be in the range of

1100 ° - 1200 °F.

We have turned our attention over to the second issue of this assessment study, having to do with the assembly of available data on potassium

ion attack (here meant to refer to hot corrosion attack by potassium

^ compounds) . The test data obtained by the Bureau of Mines-'- and in England,

are virtually the only data on potassiiim ion hot corrosion attack. These

test results were noted in the previous two quarterly progress reports.

There is some work, currently, on the effect of pure potassium vapor on

alloys, in connection with the development of the closed cycle potasssium

vapor turbine scheme for electric power generation.^ It was reported that

in potassium loop tests in the range of 1600 to 1800 °F (870-990 °C) at

Oak Ridge, -for the purpose of screening alloys for these turbines, types 304,

316, and 321 stainless steels, Inconel 600, Hastelloy X, and Haynes 25 all

resisted potassium attack. More relevant, however, is the considerable data

in the literature on hot corrosion attack by Na2S0^ and, in lieu of potassium ion

data, this has been used as a basis of selection of alloys included in the

tables of property data. Of course, hot corrosion tests should be condvicted

in the actual potassium compounds involved, i.e., K2SO4 , K2CO3 , and KOH

probably being the most important ones, before final selections are made. A

further search of the literature will be made for data on hot corrosion

by potassium ions.

Data sheets were begun in the previous quarterly report, containing

the reported values of the salient properties of certain promising alloys.

This format is continued, and data on a number of new alloys with promising

properties are included in this report. Particular attention should be paid

to the relative amounts of the various species of "carbide formers" in the

"Alloy Compositions" table (Table iv) . Recent research has revealed that

the hot corrosion pitting attack can be overcome by the substitution of Hf

and Ta for the Mo and W as carbide formers.'^ (See Figure lb). It is

anticipated that modifications of some of the commercial alloys will be

announced in the not too distant future as a result of this observation.

It is interesting to note that molybdenum has the exact opposite effect

here from its effect in another well-known type of pitting attack, i.e.,

molybdenum (precipitated as a carbide) is responsible for the pitting

attack by molten sodium sulfate, but inhibits pitting by sodium chloride

in aqueous solution.

An additional column has been added to the "Mechanical Properties" table

(Table V ) , for entering the ASME Boiler and Pressure Vessel Code maximum

allowable design stresses as a function of temperature. Many of the alloys

listed have already been approved as materials of construction for boiler

tubes, etc., in present-day power plants. In doing so the maximum allowable

stress to be used in design is specified as a function of metal wall

temperature. Another column has also been added to the same table for

listing comparative prices.

Another Table lists some of the "Thermal Properties" of these alloys

(Table VI).

The INCO alleys added in this report are relatively new alloys but

they have undergone extensive in-service testing as boiler tubing in

conventional coal-fired and oil-fired power plants. In this type of

service the producer recommends the following as maximum boiler tube

wall temperatures:

Alloy Boiler Tube Metal Wall

Max. Temperature

Inconel 517

Inconel 802

Inconel 800H

Incoloy

1260 °C

876 °C

622 °C

622 °C

There is need for more data on the resistance of the alloys to hot

corrosion. Of course cladding is a way of obtaining both high strength

together with high corrosion resistance or even aqueous chloride ion

resistance on one side and resistance to Na SO^ hot corrosion pitting on

the other. The 671/800 and the 671/800H are the only clad products in

this category of alloys that have been produced commercially but, if the

combination of high strength and maximum hot corrosion resistance is

needed, it is understood that 617 could be clad with 671. (Figure 1^.

Future Plans

Additional comparison cost figures and additional key property data on

the alloys listed in the Mechanical and Thermal Properties Tables, will be

obtained. The alloys listed thus far have been selected primarily on the

basis of the anticipated boiler tube requirements. It is not to be implied

that this list is complete as regards promising boiler tube alloys; in

addition to new alloys, many of the alloys listed have equivalent counterparts by other trade designations and of course produced by other mills.

Additional alloys will be added in future reports.

In addition, in future reports, a summary of promising haj d facing

alloys and surface treatments for valve seats will be compiled with primarily

the valves of the regenerative air preheater in mind.

References . . _ 1. D. Bienstock, R. J. Demski, R, C. Corey, "Corrosion of Heat-Exchange

Tubes in a Simv.lated Coal-Fired MHD System", Journ. of Engineering

Power, 93, Series A, No. 2, pp. 249-256 (1971) (Trans, of ASME).

2. J. B. Haywood and G. J. Womach, ed., "Open-Cycle MHD Power Generation",

c 1969, Pergamon Press.

3. W. F. Zimmerman and R. G. Frank, "Materials Requirements for the Alkali

Metal Topping Cycle", 105th AIME Annual Meeting, Las Vegas, Feb. 22-26, 1976.

4. J. H. Wood, D. A. Shores, N. R. Lindblad, "Control of MC Carbide

Composition for Improved Hot Corrosion Resistance in a Nickel Base

Superalloy" , 105th AIME Annual Meeting, Las Vegas, Feb. 22-26, 1976.

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Thermal Properties

Alloy Thermal

Conductivity

BTU/hr. /sq.ft./ F/ft.

Mean Coef. of Thermal Exp.

. xlO"^/°F

Haynes 25 15.9(1200°F) 9.4(70-1800 F)

310 S.S. 8.2(212 F) 8.8(32-212 F)

316 S.S. 9.4(212 F) 8.9(32-212 F)

446 S.S. 14.3(68-932 F) 7.4(68-1832 F)

IN713C 18.3(1600 F) 8.6(70-1600 F)

U500 14.1(1600 F) 8.9(70-1600 F)

FSX418 ...

WI52 15.8(1300 F) 8.8(70-1800 F)

INCO CLAD 671/800H

Inconel 671 9.1(70°F) 7.99(78-1200°F)

Incoloy 800

Inconel 617

Incoloy 800H

Haynes 188 12 7(1200°F) 8.6(70-1200°F)

Hastelloy X 13.3(1300''F) 8.6(79-1200°F)

I
I